Catastrophic Ocular Surface Failure in the Horse

1. Anatomy of the Ocular Surface

The ocular surface and its associated adnexal structures form an integrated functional unit that is essential for vision and ocular health.¹  The optically specialized ocular surface consists of the conjunctiva, limbus, and cornea. Normal eyelid function and a healthy precorneal tear film are also vital components of the ocular surface.

Conjunctiva

The conjunctiva is a non-keratinized, vascularized mucosal epithelium interspersed with mucin-producing goblet cells and lymphoid follicles and a connective tissue stroma that extends from the eyelid margin to the corneo-scleral limbus.¹  The conjunctival attachment to the underlying stroma provides a slight physical barrier to movement of antibodies, inflammatory cells, infectious agents, and ophthalmic medications. The conjunctival epithelium is continuous with the corneal epithelium. Conjunctival epithelial cells secrete a glycocalyx of mucopolysaccharides that coat the conjunctival and corneal surfaces to evenly distribute the precorneal tear film located anterior to the epithelial layers (Fig. 1).
 

Conjunctiva-associated lymphoid tissue is critical for ocular surface defense and includes lymphocytes, macrophages, mast cells, immunoglobulin G (IgG), IgA, and IgM. The conjunctival goblet cells produce the hydrophobic inner layer of the tear film, the mucin layer, which is responsible for precorneal tear film adherence and stability.¹

Precorneal Tear Film

The ocular surface is covered by a precorneal tear film (PTF). It lubricates the roughened corneal surface, provides oxygen and nutrition to the avascular cornea, enables epithelial cell proliferation, maturation, and movement over the ocular surface, and participates in ocular immunologic defenses. The PTF averages 7 μm in thickness (range, 6–20 μm) in humans.² The thickness of the PTF of the horse has not been determined. The architecture of the PTF is a complex, layered, mucoaqueous gradient gel.¹ The anterior most layer is the lipid or oily layer derived from secretions of the eyelid margin meibomian glands. The lipid layer is 0.1–0.5 μm thick and prevents premature evaporation of the PTF and ocular surface failure. The aqueous lacrimal gland–produced tear layer is at least 5 μm thick. The posterior mucin layer is ~1 μm thick and is derived from secretions of conjunctival goblet cells. The hydrophilic nature of mucin substantially reduces surface tension and provides a smooth, wettable surface for the aqueous tear layer. The separation between the middle lacrimal tear layer and the posterior mucin layer is not precisely distinct, making definitive thickness measurements of the two tear components difficult. It is highly likely that these two components form a graded mixture, with the posterior mucous component gradually blending into the anterior aqueous lacrimal tear component.^1,3

Lysozyme, lactoferrin, albumin, immunoglobulins, epidermal growth factor, transforming growth factors (TGFs), interleukin (IL), cytokines, glucose, antioxidants, and electrolytes are present in human tears.¹ The PTF of the horse contains IgA, IgM, IgG, and IgT,^4,5 urea,⁶ connective tissue growth factor,⁷ and various classes of proteases.⁸ The pH of horse tears is 8.33 ± 0.15.9

Equine Corneal Microanatomy/Physiology

The corneal epithelium is a transparent, highly organized, stratified squamous non-keratinized tissue consisting of differentiated cells on a basement membrane. The avascular cornea derives nutrition anteriorly from the tear film and internally from the aqueous humor. Corneal vascularization causes irregularities of the ocular surface that interferes with light refraction and increases the vulnerability of the ocular surface to immunological attack.¹  [...]